On About.com: Pregnancy & Childbirth, Robin Elise Weiss asks one of the most common questions seen repeatedly on pregnancy message boards: Can they tell how big the baby is by ultrasound? The question is usually followed by some variation of “…because I’m 36 weeks and the baby is already 8 pounds and my doctor says I’ll probably need a c-section.”

Ultrasound exams are notoriously inaccurate for predicting the weight of your baby. The ultrasound gives an estimate of the weight of your baby, but this estimate can be off a pound or more in either direction. There are multiple ways to predict the weight via ultrasound, making it not advised for most to make decisions about induction and planned cesarean section based solely on estimated fetal weight.

While it makes patients uncomfortable to think that a) care providers routinely rely on inaccurate measures of fetal weight when making recommendations of the safety of vaginal birth and that b) care provider bias plays a significant role in determining whether or not a woman will have an unnecessary cesarean, Weiss writes briefly about one study that suggests that a bedside ultrasound will increase the chance of surgical birth.

One thing that is show in some studies is that an estimation of a big baby via ultrasound can increase the likelihood that you will have a cesarean. This is thought to be because your practitioner has the “big baby” seed planted in his or her mind already and is less tolerant of variances in labor.

We sought to determine whether the overestimation of ultrasound-derived estimated fetal weight (EFW) is associated with increased diagnosis of labor arrest.

Study Design

This is a historical cohort study of nulliparous women with term pregnancies who underwent bedside ultrasound examination for EFW before labor induction. Labor outcomes of women with EFW overestimation > 15% the actual birthweight were compared with those with EFW not overestimated.

Results

Overestimation of EFW occurred in 9.5% of cases (23/241). The rate of cesarean delivery (CD) for labor arrest was higher for those with EFW overestimation (34.8% vs 13.3%; P = .01) even though there were no differences in length of the induction duration. After adjusting for confounding factors, EFW overestimation remained associated with CD for labor arrest (odds ratio, 4.8; 95% confidence interval, 1.5-15.2).

Conclusion

Our finding suggests that an overestimation of EFW may be associated with a lower threshold for CD for labor arrest.

On Weiss’ post, readers were giving the chance to weigh in on fetal weight estimates with their own experiences.

Hinting that she was small, the Doc said that at the required-to-stay-eligble-ultrasound the day before Bobbie was born, and weighed in at 10 pounds 13 ounces.

Ouch!

Estimated to be over 8.5 pounds… my baby was delivered 6lbs 6oz. My midwife (CNM) said “push hard… this is going to be a big baby”. I did and she came out in one push… and I had a 2nd degree tear.

One right, one wrong

They said my first son was going to be somewhere around 9 lbs. but wound up being 7 lbs 13 oz. With my second I had my last ultrasound at 28 weeks, they guessed he was about 5 lbs at that time. I thought this was really big and kinda laughed knowing how wrong ultrasounds can be. Funny thing is that he was born at 11 lbs. 2 oz. I guess it was a bit of a better guess with my second.

Off by a pound

My twins were supposed to be ‘within an ounce of one another’ just a bit before I went into labor. At birth, they were a full pound different. So rarely do OB’s notify their patients that 3rd trimester ultrasounds have an acceptable variance of + / - two weeks!

Off by a pound +

The ultrasound said 6lb 6 oz, He was born at 5lbs even by VBAC at home. He was a scrawny little guy, but healthy. As a doula I see the inaccuracies all the time. Unfortunitly some DRs do not let their clients know that and moms make decisions based on faulty info. I HATE THAT!

Bigger Baby

I only had one u/s at 20 weeks and then manual assessment. All along they told me my baby was very avg, maybe 7 1/2 lbs…my daughter was born at 10.5 oz. and 24”. Not unusual for in my family but maybe good that doctors didn’t use the big baby scare to push early interventions. Induction was never mentioned even though I birthed at 41.6 weeks (also very normal in our family) and there was never a mention of a possible section.

It happens!

I had an ultrasound in labor and they suggested that my baby was really big. I was told 10 lbs 13 oz and 23 in. They convinced me I needed a c-section. He wound up weighing in at 7.5 lbs. I was really sad. I turned around and had a vaginal birth with my next baby who was over 8 lbs - so the weight wasn’t an issue, it was my doctor’s concern about the weight that made me have a c-section.

Estimate Was Incorrect

My baby’s final ultrasound was two days before she was born, during a biophysical profile. The ultrasound guessed her at 8lbs and it was over a pound off! Thankfully, I had been warned by my practitioner and my childbirth educator that ultrasounds are not a good way to estimate weight. She was born vaginally with no complications and I was not induced.

Waaaay off.

I was really surprised because my doctor acted like this ultrasound was just as accurate as the scale. Because they thought my baby was about 8 lbs. they said my due date was off. So we induced. My 6 lb baby spent over a week in NICU with breathing problems. The NICU doctors said she was born too early.

To get a sense of how pervasive the recommendation of cesarean for suspected macrosomia based on inaccurate ultrasound measurements has become, set up a Google Alert for “ultrasound big baby c-section” or “ultrasound big baby cesarean,” or perform a Google search for the sameterms.

If you had an ultrasound estimate of fetal weight performed during your pregnancy, how did it compare to actual birth weight?

Reader Comments (20)

My OB sent me to the sonographer for suspected "big baby" one day before my EDD, certain I was having a 10lb'er. The result was that my baby was 7lb, 7oz. When she was born (I went into labor the next day but she was born via "emergency" cesarean, she was 8lb, 8oz. In my mind, my OB was just looking for a reason to cut me, and I wonder if the sonographer was trying to spare me (knowing what I know now, he's quite knife-happy.) I still got cut, but it was a pit-to-distress situation.

You brought up an important point that ultrasounds, fundal measurements and Leopold maneuvers are all subjective. There is a human using an ultrasound to make these measurements. I think a lot of people forget that because it comes from a machine and a print-out is given but it's still an estimate based on measurements made by a human being.

Realistically, ultrasounds do not need to be done during pregnancy. They have been shown time and time again to not change the outcome of the pregnancy. Most insurance companies will pay for one ultrasound during pregnancy so they are ordered, mainly because physicians do not want to miss a major anomaly, but because patients want them. Why do patients want ultrasound? Most of the time, it is because they want to know the sex of the baby, and that is not an indication for an ultrasound. I have patients constantly asking for a second ultrasound because the tech couldn't tell them "for sure" and their baby shower is coming up. We have become flippant with ultrasounds. Ultrasounds are not subjective, they are statistical but they are only as good as the technician performing the test, and they have a 10% error estimating fetal weight, in both directions. This could mean at least a pound off in either direction. The basic premise is this, if I told you how much a car weighed, could you tell me if it was going to fir through the garage door. Fetal weight has nothing to do with delivery success, and it should be stated as such. Suspected macrosomia is also not an indication for induction. OBs that I know are not looking for a reason to cut people, that to me is ludicrous, but I imagine it is possible. Ultrasounds are error prone, and they predict nothing when it comes to the success of a delivery.

My last child was born August 2007. I was 41 and have type II diabetes so I had ultrasounds about once a week toward the end of my pregnancy. At 37 weeks, my OB said the baby was too big and that at the rate she was growing, she'd be 12 lbs if he let me go to 40 weeks and I had to have a section. I tried to find another doctor but no one wanted to take on a diabetic, over 40 mother with only a few weeks to go. The last ultrasound I had at 38.5 weeks, estimated the baby to be 10 lb. 12 oz. When I had my unnecesarean at 39 weeks, she weighed 9 lb. 14 oz. My second child was also estimated to be a pound more than she weighed at birth.

With DS#2 I had an u/s at 35 weeks because my midwife was afraid he was breech. The profile listed his weight at being in the 45th percentile. At birth (he was born on his due date) he weighed 9lbs 8oz -- the 97th percentile.

Maybe it's just that I had a CNM (although she turned out to be kinda a "med-wife", didn't show up until I was 6 cm, the OB on call had broken my water, I'd caved for an epi, was on pit, and I was heading into deep transverse arrest and the OR...)

Anyway, point is, I had a 20 week ultrasound and they never tried to estimate the baby's weight, as far as I know. She was 6lb 12 oz but it didn't matter. Her head was sideways, they couldn't turn her. Don't know if it's my own fault for agreeing to interventions or if it was happening anyway, but now I'm just hoping like hell I can find a vbac provider.